This invention relates to improvements in and concerning the quality of a silicon semiconductor substrate, more particularly to a silicon semiconductor substrate which allows exclusion of a defect from the interior or surface of the substrate and permits formation of devices on the substrate in an exalted yield, and a method for the production thereof.
This invention relates further to improvements in and concerning the quality of an epitaxial silicon semiconductor substrate, more particularly to an epitaxial semiconductor substrate which, owing to the exclusion of a defect from the epitaxial layer and from the neighborhood of the interface between the epitaxial layer and a substrate wafer and the improvement in gettering ability, permits formation of devices on the epitaxial substrate in an exalted yield, and a method for the production thereof.
It is known that in consequence of the improvement attained in recent years in the degree of integration of devices, minute defects existing on the surface and near the surface layer of a silicon semiconductor substrate since immediately after manufacture of the substrate and crystal defects induced during the course of production of devices result in lowering the yield of production of devices as by inducing impairment of device patterns during the formation of devices and imperfecting the devices in performance. As the cause for this lowered yield of devices, the defects called xe2x80x9ccrystal originated particlesxe2x80x9d or xe2x80x9ccrystal originated pitsxe2x80x9d (COP) that are detected in the form of pits, about 0.1 xcexcm in size, on the surface of a substrate immediately after manufacture of the substrate have been attracting attention.
The reason for calling the defects by this name is that when a silicon semiconductor substrate is etched with an ammonia-hydrogen peroxide liquid mixture, pits which originate in lattice defects in a crystal manifest themselves on the surface of the substrate and these pits are detected during the determination with a testing device adapted to take count of particles on the substrate surface. The COP is a term for designating all the defects that are detected by such method of determination. In the single silicon crystal which is grown by the ordinary Czochralski (CZ) method or the CZ method using an applied magnetic field, these defects are considered to be actually octahedral voids (hereinafter referred to as xe2x80x9cvoid defectsxe2x80x9d) in the crystal. It is inferred that these void defects induce the devices to suffer from impairment of patterns or compel the devices to incur structural breakage. To date, several techniques have been proposed with a view to decreasing or disappearing the COP which is harmful to the manufacture of devices.
As a technique for disappearing the COP, the method which consists in limiting the speed of growth of a single crystal to not more than 0.8 mm/min (JP-A-02-267,195) has been known. This method is intended to repress the occurrence of supersaturated vacancy-type point defects (vacancies) in a single crystal being grown during the course of cooling by decreasing the amount of vacancies, i.e. an element which gives rise to void defects, to be introduced into the interface of growth of the crystal and slackening the speed of cooling the crystal as well. This method, however, entails such problems as deriving a decrease in productivity from a fall in the speed of growth and, at the same time, generating such crystal defects as dislocation loops which are different in kind from the COP.
As techniques for repressing the generation of COP, the method of controlling the behavior of a single crystal in the course of cooling and particularly the method of controlling the time required by the single crystal in passing an approximate range of temperature from 1200xc2x0 C. to 1000xc2x0 C. have been known to be effective (JP-A-08-12,493, JP-A-08-91,983, and JP-A-09-227,289). These techniques pose no problem in terms of productivity because they bring no noticeable decrease in the speed of growth of the single crystal. Since they have the lower limits of decrease of the density of COP generally in the neighborhood of 105 pieces/cm3, they encounter difficulty in attaining a further decrease in the density to not more than 104 pieces/cm3, for example.
As a technique for decreasing the COP, the method which comprises limiting the time for retaining a single crystal, while the crystal is in the process of being cooled during the growth thereof, in a temperature range of 850xc2x0 C.-1100xc2x0 C. to less than 80 minutes or attaining in the growth of a crystal a silicon single crystal having a nitrogen concentration of 1xc3x971014/cm3 and thereafter manufacturing the silicon single crystal into a silicon wafer and heat-treating the wafer at a temperature of not less than 1000xc2x0 C. for not less than one hour (JP-A-10-98,047). This technique is intended to abolish defects during the heat treatment by shifting the size distribution of the COP generated during the production of the crystal toward the smaller side. Since it is generally held that the effect of this decrease in size gains in prominence in accordance as the oxygen concentration decreases, however, this technique is not put to use at the oxygen concentration of 7xc3x971017-10xc3x971017/cm3 which is commonly used in the Czochralski method. Thus, it is difficult to establish compatibility between the impartation of the gettering ability which utilizes the formation of an oxygen precipitate in the substrate generally attained by increasing the oxygen concentration in the substrate and the decrease of the COP.
In addition to the technique for decreasing the COP during the growth of a single crystal, the technique which effects decrease or disappearance of the COP on the surface of a substrate by slicing and polishing a single crystal and manufacturing it into a substrate and thereafter subjecting the substrate to a heat treatment has been known. JP-A-03-233,936, for example, proposes the performance of a heat treatment at 800-1250xc2x0 C. for not more than 10 hours. When the heat treatment is carried out in an oxidizing atmosphere indicated in a working example which is cited in this patent publication, however, it is at a disadvantage in inducing an increase in the number of pits formed on the surface of the substrate because the etching by oxidation of the surface of the substrate entails etch pits of void defects to the surface of the substrate and, at the same time, rendering it difficult to lower the density of COP within a depth of 1 xcexcm from the surface of the substrate below 104 pieces/cm3. Then, JP-A-59-202,640 proposes the performance of a heat treatment in an atmosphere of hydrogen. Though this method, owing to the use of the atmosphere of hydrogen, is capable of abolishing the COP on the outermost surface and lowering the COP density within a depth of 0.5 xcexcm from the surface below 104 pieces/cm3, it is incapable of lowering the density of the COP in a deeper portion from the surface below 104 pieces/cm3 and unsatisfactory for the formation of a defectless layer from the viewpoint of the manufacture of devices. Moreover, this method uses an explosive atmosphere of hydrogen and, therefore, requires a perfect measure for safety.
Concerning the doping of nitrogen preparatory to the growth of a single crystal of silicon, methods for the doping have been known as from JP-A-60-251,190, etc. In search of the effect of doping of nitrogen on the floating zone (FZ) single crystal, JP-A-57-17,497 discloses a method for enhancing the strength of crystal and JP-A-08-91,993 a method for repressing variation in the resistance. Then, JP-A-05-294,780 has a disclosure to the effect that the nitrogen doped into silicon acts on or binds with vacancies, i.e. one form of complexes, and consequently represses the formation of vacancy participated clusters (void defects) and curbs the occurrence of etch pits which are thought to originate in void defects. It has been reported by D. Graf et al. (The Electrochemical Society Proceeding, Vol. 96-13, pp. 117, 1996) that where oxygen is present in a single crystal, the doping of nitrogen results in decreasing the COP defects. They explain this mechanism by inferring that a mechanism similar to one which represses vacancies in the FZ crystal functions in the case of a CZ crystal and decreases the size of void defects which are aggregates of vacancies. As reported in K. Kakumoto, et al.; Proceedings of The 2nd International Symposium on Advanced Science and Technology of Silicon Materials, p. 437-442 (1996), it has been known that when the defects resulting from the linkage of nitrogen and vacancies increase, they constitute themselves the centers of generation or recombination of electrons and electron holes in silicon crystal and alter electric properties and also that in a silicon substrate containing oxygen, nitrogen forms compound defects with oxygen and consequently alters the resistance of the substrate and further facilitates the occurrence of stacking faults in consequence of a heat treatment.
Further, as a silicon semiconductor substrate which inhibits occurrence of various crystal defects such as oxygen precipitate, dislocation loop, and stacking faults other than the COP in the neighborhood of the surface of the substrate, the epitaxial silicon substrate which is produced by epitaxially growing a silicon single crystal layer as by the method of chemical vapor growth on a wafer manufactured by slicing and specular polishing a silicon single crystal grown by the CZ method or the CZ method using application of a magnetic field has come to attract attention and find adoption in actual use.
Though the epitaxial silicon substrate is a substrate which has newly deposited on a silicon wafer a single crystal layer such that contains substantially no oxygen or defect as described above, it has entailed such problems as generating defects in the epitaxial layer, depending on the surface condition of the wafer during the deposition of the epitaxial layer (the presence of void defects such as the COP, defects such as pits originating in oxygen precipitation and minute hills called hill rocks, and stacking faults), inducing defects in the epitaxial layer, or generating defects in the epitaxial layer during device fabrication process because of void defects or oxygen impurities existed in the neighborhood of the wafer surface, or inducing defects owing to diffusion of vacancies or oxygen impurities into the epitaxial layer. For the purpose of obtaining an epitaxial silicon substrate of high quality, therefore, the technique for attaining thorough preclusion of defects on the surface and in the neighborhood of the original wafer itself which is fated to support the epitaxial layer thereon plays an important role. Since the wafer itself is required to have a gettering ability for the sake of offering resistance to various forms of pollution liable to occur during the course of manufacture of devices, it is likewise important to have defects having the gettering ability built in suitably in the central part of the wafer. Further, the cost of production of the substrate tends to increase because the process of production from the growth of a single crystal till the impartation of an epitaxial layer is long and because the quality control is carried out strictly. It is also an important task to find an answer to the question how an epitaxial silicon semiconductor substrate of high quality is to be produced at a low cost.
Concerning silicon wafers which are used for epitaxial substrates, several inventions published to date offer techniques for decreasing defects in the neighborhood of surface and techniques for building defects in the wafer interior for the sake of improving the gettering ability. JP-A-05-283,350, for example, proposes a method for the production of an epitaxial silicon semiconductor wafer, which comprises subjecting a wafer which has undergone an intrinsic gettering (IG) treatment to a heat treatment in an atmosphere containing hydrogen prior to the vapor growth of a silicon single crystal thin film thereby abolishing from the wafer-substrate the points of origin for the occurrence of defects in the silicon single crystal thin film and thereafter forming a thin film by vapor growth. Then, JP-A-08-250,506 proposes a silicon epitaxial wafer having a BMD density adjusting region formed in the interior thereby, obtained by using a wafer manufactured from a silicon ingot and subjecting this wafer to a step of performing an IG treatment for the impartation of IG effect, a step of retaining a temperature for controlling the density of oxygen precipitate (BMD), and a step of performing an epitaxial treatment on the wafer surface. Further, JP-A-09-199,507 proposes a semiconductor substrate which, by a specific heat treatment serving to effect inclusion of a stated amount of a SiO2 precipitate in a deeper layer than the LSI forming zone where the surface layer has a denuded zone (DZ) and to effect inclusion of a stated amount of SiO2 precipitate substantially uniformly from the surface downward where the subsequent step forms an epitaxial film, is made to exhibit a high IG ability to polluting heavy metals, decrease the warp of a substrate, and excel in strength. Since these techniques, while necessitating varying forms of heat treatment, attach the foremost priority to securing the IG effect of the wafer""s own, they entail such problems as failing to attain perfect elimination of the crystal defects existing on the surface and near the surface layer of a substrate wafer fated to base deposition of an epitaxial layer and doing harm to epitaxial growth, suffering persistence of defects in the epitaxial layer, and inducing the occurrence of defects in the process of manufacture of devices. The heat treatment itself is at a disadvantage as well in being so complicated as to degrade productivity greatly and boost the cost of production. Then, JP-A-08-162,406 proposes a wafer which is obtained by preparatorily subjecting a substrate silicon wafer containing crystal defects at a high density of not less than 5xc3x97106 pieces/cm3 to epitaxial growth thereby providing a gettering layer in the interior of the substrate. This method likewise entails such problems as failing to attain perfect elimination of the crystal defects existing on the surface and near the surface layer of a substrate wafer fated to base deposition of an epitaxial layer and doing harm to epitaxial growth, suffering persistence of defects in the epitaxial layer, and inducing the occurrence of defects in the process of manufacture of devices.
The conventional techniques combine merits and demerits of their own as described above. With a view to answering the demand imposed in recent years on the semiconductor devices to attain further miniaturization and integration, therefore, the desirability of developing a method for effecting elimination of the crystal defects near the surface of a silicon substrate so as to permit provision at a low cost of a semiconductor substrate of such high quality as has a fully satisfactory IG ability in the substrate has been finding growing recognition.
In the first aspect, this invention has an object of providing a silicon semiconductor substrate having decreased or disappeared with high productivity the crystal defects which occur on a silicon semiconductor substrate for use in the manufacture of semiconductor devices, can not be abolished completely by the conventional techniques mentioned above, and pose a problem to the manufacture of devices thereon, and a method for the production thereof.
This invention, in the formation of a DZ by the heat treatment of a silicon single crystal substrate for use in the manufacture of semiconductor devices, has a further object of providing, by virtue of a heat treatment using a safe atmosphere, a semiconductor substrate incorporating therein a DZ of high quality not having such crystal defects as COP copiously.
This invention in the second aspect has an object of providing an epitaxial silicon semiconductor substrate of high quality and low cost which has generously repressed the occurrence of defects in the epitaxial layer and in the region approximating the interface between the epitaxial layer and a substrate wafer and has imparted an IG characteristic to the epitaxial layer and a method for the production thereof.
The present inventors, after pursuing a diligent study concerning the defects which occur in a silicon semiconductor substrate, have discovered that the defects of a size such as to pose a problem in the region of manufacture of devices on a silicon semiconductor substrate can be abolished nearly completely. This invention has been perfected based on this discovery.
Specifically, in the first aspect, this invention concerns a silicon semiconductor substrate which is obtained from a silicon single crystal as grown by the Czochralski (CZ) method and is characterized by the fact that in a region reaching a depth of at least 1 xcexcm from the surface of the substrate, the density of crystal defects measuring not less than 0.1 xcexcm as reduced to diameter is not more than 104 pieces/cm3. More preferably, the silicon semiconductor substrate has at the center of thickness a nitrogen content of not less than 1xc3x971013 atoms/cm3 and not more than 1xc3x971016 atoms/cm3. Further, the silicon semiconductor substrate according to this invention is such that the nitrogen content thereof is not more than 1xc3x971016 atoms/cm3, particularly not less than 1xc3x971013atoms/cm3 and not more than 1xc3x971016atoms/cm3, and the nitrogen concentration thereof determined throughout the entire volume of the substrate by the method of secondary ion mass analysis has a part locally concentrated by a nitrogen segregation exhibiting a signal intensity of not less than twice the average signal intensity.
This invention further concerns a silicon semiconductor substrate which is obtained from a silicon single crystal grown by the CZ method and is characterized by containing crystal defects at a density distribution decreasing from the center of thickness of the substrate to the surface, containing crystal defects measuring not less than 0.1 xcexcm as reduced to diameter on the surface of the substrate at a surface density of not more than one piece/cm3, containing crystal defects measuring not less than 0.1 xcexcm as reduced to diameter at a depth of 0.1 xcexcm from the surface of the substrate at a volumetric density of not more than 1% of that at the center of thickness of the substrate, and having at the center of thickness of the substrate a nitrogen content of not less than 1xc3x971013 atoms/cm3 and not more than 1xc3x971016 atoms/cm3. The term xe2x80x9ccrystal defectsxe2x80x9d as used herein embraces all the crystal defects such as void defects, oxygen precipitate, and stacking faults which are causes for defective devices.
This invention further concerns a method for the production of a silicon semiconductor substrate characterized by heat-treating at a temperature of not less than 1000xc2x0 C. and not more than 1300xc2x0 C. for not less than one hour a silicon semiconductor substrate obtained from a silicon single crystal grown by the CZ method using fused silicon containing nitrogen at a concentration of not less than 1xc3x971016 atoms/cm3 and not more than 1.5xc3x971019 atoms/cm3. Further, the growth of the silicon single crystal by the CZ method is preferred to proceed under conditions satisfying the formula, V/Gxe2x89xa70.2 (mm2/xc2x0C.min), wherein V denotes the speed of pulling (mm/min) and G the average of the temperature gradient in the crystal in the direction of the axis of pulling in a temperature range from the melting point of silicon to 1300xc2x0 C. (xc2x0C./mm). Preferably, the heat treatment is carried out in a non-oxidizing gaseous atmosphere or it is carried out in a gaseous atmosphere containing not less than 0.01 vol. % and not more than 100 vol. % of oxygen, with the surface of the substrate subsequently polished to a depth of not less than 0.5 xcexcm and not less than 1.0 xcexcm till specular finish.
Concerning the defects which occur on an epitaxial silicon semiconductor substrate, the present inventors have pursued a diligent study on such experiments and theoretical discussions as covering the step of manufacturing devices, the step of epitaxial growth, and further the step of production of a silicon wafer and have consequently acquired a novel knowledge. This invention has been perfected as a result.
Specifically, in the second aspect, this invention concerns a silicon semiconductor substrate characterized by using a silicon wafer having a nitrogen content of not less than 1.0xc3x97103 atoms/cm3 and not more than 1.0xc3x971016 atoms/cm3 as a substrate wafer and depositing on the surface of the substrate wafer a silicon single crystal layer by the epitaxial method.
This invention further concerns a silicon semiconductor substrate which is obtained by using a silicon wafer having an oxygen content of not less than 1.0xc3x971017 atoms/cm3 as a substrate wafer and depositing on the surface of the substrate wafer a silicon single crystal layer by the epitaxial method and is characterized by containing crystal defects measuring not less than 0.1 xcexcm as reduced to diameter at least in a region reaching a depth of 1 xcexcm from the interface between the substrate wafer mentioned above and the silicon single crystal layer deposited by the epitaxial method at a density of not more than 5xc3x97104 pieces/cm3. Preferably, the substrate wafer mentioned above contains not less than 1.0xc3x971013 atoms/cm3 and not more than 1.0xc3x971016 atoms/cm3 of nitrogen.
This invention further concerns a silicon semiconductor substrate which is obtained by using a silicon wafer having an oxygen content of not less than 1.0xc3x971017 atoms/cm3 as a substrate wafer and depositing on the surface of the substrate wafer a silicon single crystal layer by the epitaxial method and is characterized by containing crystal defects measuring not less than 20 nm as reduced to diameter at least in a region reaching a depth of 1 xcexcm from the interface between the substrate wafer mentioned above and the silicon single crystal layer deposited by the epitaxial method at a density of not more than 5xc3x97105 pieces/cm3. Further, in the region reaching a depth of 1 xcexcm from the interface between the substrate wafer mentioned above and the silicon single crystal layer deposited by the epitaxial method, the density of the crystal defects measuring not less than 0.1 xcexcm as reduced to diameter is preferred to be not more than 5xc3x97104 pieces/cm3. Further, the substrate wafer mentioned above is preferred to contain not less than 1.0xc3x971013 atoms/cm3 and not more than 1.0xc3x971016 atoms/cm3.
Further, in any of the silicon semiconductor substrates according to the second aspect of this invention, the crystal defects measuring not less than 20 nm as reduced to diameter are preferred to be contained at the center of thickness of the substrate wafer at a density of not less than 1xc3x97108 pieces/cm3.
This invention further concerns a method for the production of a silicon semiconductor substrate characterized by using a silicon wafer obtained from a silicon single crystal grown by using molten silicon containing not less than 1.0xc3x971016 atoms/cm3 and not less than 1.5xc3x971019 atoms/cm3 of nitrogen as a substrate wafer and depositing on the surface of the substrate water a silicon single crystal layer by the epitaxial method.
Further, this method concerns a method for the production of a silicon semiconductor substrate characterized by using a silicon wafer obtained from a silicon single crystal grown by the Czochralski method at a cooling speed of not more than 2.0xc2x0 C./minute through the range of temperature from the solidifying point to the crystallization point of 800xc2x0 C. as a substrate wafer and depositing on the surface of the substrate wafer a silicon single crystal layer by the epitaxial method. Further, the silicon single crystal is preferred to be grown by using molten silicon containing not less than 1.0xc3x971016 atoms/cm3 and not more than 1.5xc3x971019 atoms/cm3 of nitrogen.
This invention further concerns a method for the production of a silicon semiconductor substrate characterized by using a silicon wafer obtained from a silicon single crystal grown by the Czochralski method at a cooling speed of not less than 1.0xc2x0 C./minute through the range of temperature from 800xc2x0 C. to the crystallization point of 400xc2x0 C. as a substrate wafer and depositing a silicon single crystal layer on the surface of the substrate wafer by the epitaxial method. Further, the silicon single crystal is preferred to be grown by using molten silicon containing not less than 1.0xc3x971016 atoms/cm3 and not more than 1.5xc3x971019 atoms/cm3 of nitrogen.
Then, this invention concerns a method for the production of a silicon semiconductor substrate which is characterized by using as a substrate wafer a silicon wafer obtained from a silicon single crystal grown by the Czochralski method, i.e. the silicon single crystal grown during the course of pulling the crystal at a cooling speed of not less than 2.0xc2x0 C./minute through the range of temperature from the solidifying point to the crystallization point of 800xc2x0 C. and subsequently grown at a cooling speed of not less than 1.0xc2x0 C. through the range of temperature of 800xc2x0 C.-400xc2x0 C., and depositing on the surface of the substrate wafer a silicon single crystal layer by the epitaxial method. Further, the silicon single crystal is preferred to be grown by using molten silicon containing not less than 1.0xc3x971016 atoms/cm3 and not more than 1.5xc3x971019 atoms/cm3 of nitrogen.
Preferably, any of the methods of production according to the second aspect of this invention comprises using as a substrate wafer a silicon wafer obtained from a silicon single crystal grown by the Czochralski method and heat-treated at a temperature of not less than 1000xc2x0 C. and not more than 1300xc2x0 C. for not less than five minutes and depositing a silicon single crystal layer on the surface of this substrate wafer by the epitaxial method.